 The central dome was then often surrounded by structures at the four points of the compass producing a cruciform shape - these were themselves often topped by towers or domes.The centralised and basilica structures were sometimes combined as in the church of Hagia Sophia in Constantinople.The basilican east end then allowed for the erection of an iconostasis, a screen on which icons are hung and which conceals the altar from the worshippers except at those points in the liturgy when its doors are opened.